Why not try film prop supply companys?
I love this post, even if it's a joke. Are you doing the 1812 Overture? If you can't find a small to medium sized cannon, you should try a more modern, shoulder-launched type of percussive instrument like an RPG. They are lots of fun for an indoor venue. Maybe call AC/DC, I think they used to have a cannon. Best of luck, David :D

Thanks for the reply Doc. No joke. need a cannon. we are doing "Civil War Fantasy" by H. Bilik. has one cannon shot in it, music director wants to put the cannon up in the balcony so it has to be kinda small. I will try some film prop places. thanks again.

Before you get too deep into this, you may want to look into local ordinances concerning public use of explosive devices. Just FIY, since I've been involved in this kind of thing before.

Max, Glad you realized was trying to give you a solid tip on the film prop co. It's just such a funny topic, can't help the jokes, let us know how it goes. David

If you want a real cannon try Dixie Gun Works. They have all sorts of equipment for historic re-enactment as well as all the "black powder" needs you can think of.
Also try your local yacht club (if you have one) or a Marine equipment supplier. Very small, yet loud cannons are used to start yacht races.

Thanks again for the help. Found a guy that was shooting off his cannon every time a touchdown was scored at a high school football game close by. He said he would love to do it. Even has the cannon hooked up to go off electronically. There will be boom at the concert if there are not any ordinances against this kind of thing. Stage manager is checking into it. Thanks again.